All excellent points Bruce and I might add to any beginners out there,
the reason I went the way I did with my antennas back when I had my
full blown satellite rig is that I "wanted" the challenge of building a
homebrew AZ/EL setup. I did enough research to know that it certainly
wasn't necessary for LEO's, omni's are fine. I'm considering homebrewing
some eggbeaters, groundplanes and turnstiles just to experiment with. I
guess I got it from my late father ( the original W4HIJ), I like to play
with antennas. :-)
Michael
Bruce Robertson wrote
>> I'm going to elaborate on this discussion, for the benefit of
>> beginners who are considering building new stations with tracking
>> antennas. The narrower the beam, of course, the greater the gain when
>> pointing at the bird, both transmit and receive, and -- this is the
>> critical issue -- the lower the gain when you are pointed away from
>> the bird. Now with LEO satellites, some of which cross the sky in say
>> 15 minutes, you need to have everything spot-on if your beamwidth is
>> very narrow: the station clock has to be accurate within a second, the
>> keps have to be up-to-date, etc. otherwise, your computer is telling
>> the rotors to point in the sky where the satellite is going to be in
>> five seconds, or will be in five. Especially with high passes, you can
>> be off by enough to not be able to hear the bird at all. So on
>> receive, long antennas, besides the additional expense and challenge
>> of mounting them, also add the challenge of getting your station
>> perfectly aligned, or you'll hear zippo.
>>>> On transmit, long antennas present another challenge: they
>> 'concentrate' your signal so that it might well be excessively
>> powerful for the satellite in question. If my homebrew 7 - element
>> 70cm yagi often needs to be down around 5w xmit on VO-52 to be in the
>> right range of effective power, how will I deal with things when I
>> have a 40-element beam? By all rights, I should put an attenuator
>> between the rig and the antenna so that I can get down under a watt!
>> It is my opinion, in fact, that a significant proportion of the
>> over-powered signals on our birds are from people in just this
>> situation: people using HEO antenna systems that simply can't provide
>> a small enough signal!
>>>> In fact, LEO satellites do not require these sorts of antenna systems
>> for reliable use. A beginner will be perfectly happy with, say, four
>> elements on 2m and 6-7 on 70cm (assuming the use of low-noise preamps,
>> which you are *crazy*to do without on long antennas, too). The beauty
>> of this system is that if a high wind knocks it slightly out of whack
>> in azimuth, it will not be the end of your satellite work: you'll just
>> have weaker signals, not silence. The other beauty of this system is
>> that it doesn't require an elevation rotor *at all*. Because the
>> elevation pattern of the antennas will more-or-less fill the sky if
>> you point the array up about 10-20 degrees (make it 10 if you have a
>> clear horizon). Now, suddenly, you've avoided all the hassle of
>> another rotor, you've made your array lighter and easier to work with,
>> and you have way less of a demand on your pointing system. Heck, if
>> you want to go ol' school, you can do the pointing yourself with a
>> twist of the dial.
>>>> These yagis do not need to be brilliantly built: mine were made with
>> welding rod and pine wood. They had very strange lobes off the side,
>> and all the rest, but they netted me lots of Q's and were very
>> reliable.
>>>> To be even more radical, I urge beginners to start with
>> omni-directional antennas and low-noise preamps. A wire dipole or a
>> vertical, both with almost no coax between them and the preamp, should
>> hear 'stuff' really well. Not Q-5, but a start. Then use this as a
>> baseline from which to compare the theoretical and real-world
>> improvement you get with your yagi array. If you aren't getting
>> improvement, then work out what's up.
>>>> This is not an argument against long arrays. I'm building some that I
>> bought from someone on this list around this time last year. I want to
>> do some exotic stuff like work Russia over the pole on AO-07 or hear
>> every last beep out of the newest cubesat. But I'm aware that in my
>> windy region these are going to be a bear to keep in place. So I'm
>> putting as much work into an omni array, too. I plan to transmit from
>> the latter when things get too QRO.
>>>> I guess in summary I'd say that in my opinion a big antenna array
>> isn't like a high-power computer, which works the same as a
>> lower-powered one, but has the umph when you need it; it is like
>> buying a high-powered plane as a new pilot: significantly more
>> challenging, and possibly leading to frustration.
>>>> 73, Bruce
>> VE9QRP
>>>>